Habit modification and preparation of single crystal triglycine sulfate



Nov. 14. 1967 c. R. PARKERSON ETAL 3,

HABIT MODIFICATION AND PREPARATION OF SINGLE CRYSTAL TRIGLYCINE SULFATEFiled Feb. 4. 1963 ATTORNEY United States Patent 3,352,906 HABITMODIFICATION AND PREPARATION OF SINGLE CRYSTAL TRIGLYCINE SULFATECharles R. Parkerson, Arlington, and Harry H. Wieder,

Riverside, Calif., assignors to the United States of America asrepresented by the Secretary of the Navy Filed Feb. 4, 1963, Ser. No.256,175 4 Claims. (Cl. 260--534) The invention herein described may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes Without the payment of any royalitiesthereon or therefor.

The present invention relates to the method of growing crystals and moreparticularly to crystal growth with controlled habit modification oftriglycine sulfate by means of monovalent cations.

A main purpose of this invention is the growth, from an aqueous solutionof triglycine sulfate and deuterium doped triglycine sulfate, of singlecrystals having a particularly highly desirable crystal habit for use inthe fabrication of better ferroelectric memory elements as well asferroelectric capacitors and dielectric modulators.

Ferroelectricity in triglycine sulfate was discovered by Matthias etal., Physical Review, 104, 849 (1956). In order to prepare ferroelectricmemory elements or other devices it is necessary to cleave a parentcrystal in a direction perpendicular to the ferroelectric axis andthereafter apply electrodes of evaporated gold or silver to oppositefaces of the thin laminae cleaved therefrom. Generally, the electrodeareas are of the order of one square millimeter. Crystals of triglycinesulfate grown from aqueous solution have an external habit, and, in viewof the large size of a parent crystal, it is difficult to cleavehomogenous sections without cleavage steps. Further, in manyapplications it is also desirable to construct a series of ferroelectricmemory elements on a series of slabs having identical geometry, and thisrequires additional processing of cleaved specimens.

In many cases the existence of an angular dispersion of theferroelectric axis in crystals of triglycine sulfate has made thecrystals unsuitable for device applications. Growth of triglycinesulfate crystals by the usual prior known methods is generally notsatisfactory because of problems in uniformity, or ferroelectricactivity which limits the use of the grown crystal volume to aboutpercent of its total due to dispersion of the ferroelectric axes.

The present invention is a significant development in the growing ofcrystals having a prefered crystallographic orientation for thefabrication of ferroelectric memory matrices. The present method ofgrowing crystals allows about 60 to 70 percent of the crystal to be usedsuccessfully for building small ferroelectric matrices. This method isnot restricted to triglycine sulfate but may prove to be useful forisomorphous crystalline solids.

It is an object of the invention therefore to provide a novel method forgrowing ferroelectric crystals.

It is a further object of the invention to provide a novel method ofgrowing crystals having greater uniformity.

Another object of the invention is to provide crystal growth withcontrolled habit modification by means of monovalent cations.

Still another object of the invention is to provide growth of singlecrystal plates with low dispersion of the ferroelectric axis.

Other objects and many of the attendant advantages of this inventionwill become readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

3,352,536 Patented Nov. 14, 1967 FIG. 1 is an illustration of a typicaltriglycine sulfate.

crystal grown by prior known method from aqueous solution and having anormal external habit as shown, and labelled with crystallographic axes.

FIG. 2 is an illustration of a typical triglycine sulfate crystal grownby the present method with controlled modification of the external habitas shown.

FIG. 3 is an edge view of the thin flat crystal shown in FIG. 2 and alsoillustrating the seed crystal and mounting rod.

As illustrated in FIG. 1, the typical crystal of triglycine sulfategrown from aqueous solution has an external habit as shown. Thecrystallographic axes are labelled for a crystal having such a habit.The axes shown are normal nomenclature for monoclinic crystals. Thelabeling of the axes is consistent with the class of monoclinic crystalsas tabulated in 'Danas Manual of Mineralogy, Wiley (1959). It is wellknown that in triglycine sulfate the b-axis is the ferroelectric axis.

The method of the present invention is for the modification of theexternal habit of triglycine sulfate and deuterated triglycine sulfatecrystals in order to obtain crystals which grow uniformly in alldirections except the crystallographic [001] direction, along theC-axis. The thickness of the crystals grOWn by the present methoddepends upon the thickness of the original seed crystal; a negligible,less than 1 percent, growth occurs in the [001] direction, along theC-axis; thus thin single crystal plates of the order of 1 millimeter to1 centimeter thickness can be obtained. Thin single crystal plates ofmore than three inches in diameter have been grown by this method suchas shown in FIGS. 2 and 3. These thin plates are easily cleaved intosections by the application of transientv pressure in the directionperpendicular to the b-axis. The b-axis, or crystallographic [010]direction lies in the plane of the crystal plate and its directionwithin the plate has a much smaller dispersion than in triglycinesulfate crystals having the normal (unmodified) habit.

The electrical properties of crystal laminae obtained from habitmodified triglycine crystals of the present invention show that they areessentially identical to those of the normal habit; however, a muchsmaller angular dispersion of the ferroelectric axis (b-axis), was foundwithin each group as compared to that of normal habit triglycinesulfate. In addition, the simplified method of preparing a ferroelectricmemory element apparently leads to a smaller number of dislocations orother defects induced during the usual cutting, cleaving, grinding andpolishing of the normal habit triglycine sulfate crystal.

Details of growing the modified habit triglycine sulfate are asfollows:.

It was found while attempting to dope triglycine sulfate with Tl+ thatthe crystal habit was very much affected by the presence of this ionwhen there is 1 gm.-atom Tl+ per 6.13 gm.-mols of triglycine sulfate insolution (10.3 gm. Tl+ per gm. triglycine sulfate in solution). Thetriglycine sulfate grows as thin plates on the [101] plane of theorystal. Seed crystals planted in this solution assume essentially twodimensional growth.

No attempt has been made to determine the exact concentration of TH ionat which this effect on crystal habit begins to take place. It wasfound, however, that the effect is not present at a concentration as lowas 1 gm.- atom Tl+: 1235 gm.-mols triglycine sulfate in solution (0.053gm. Tl+zl00 gm. triglycine sulfate in solution). The maximum effect isstill present at 1 gm.-atom Tl+:l2.26 gm.-mols triglycine sulfate insolution (5.15 gm. Tl+ per 100 gm. triglycine sulfate in solution). Astudy was made of the effect of all the monovalent alkali metal ions LiNa' K Rb+ and Cs+ using the same concentration in each case (1 gm.-atomalkali-metal ion per 6.13 gm.-mols triglycine sulfate in solution). Theenhanced [101] effect on triglycine sulfate increases as the ionicradius of the ion increases. Li+ has very little effect even atconcentrations as high as 3 gm.-atms Li+ per 6.13 grn=rnols triglycinesulfate in solution. R-b Cs+ and Tl" each give the maximum effect. Othermonovalent cations included in the study were NHJ, N(CH and Ag+.Approximately twice the concentration of NHJ or N(CH is required toenhance the [101] faces to approximately the same degree as K Ag+present to the saturation point for Ag SO in the triglycine sulfatesolution at 46 C. causes a very small enhancement of the [101] faces.Several other positive ions were studied in regard to their effect ontriglycine sulfate crystal habit. Those studied include Cu++, Cd+ VO UOIn+++ and Er+++. None of these ions exhibited any effect similar to thatcaused by the monovalent ions re ported above. Cd++, Er+++ and In+++show practically no effect on triglycine sulfate crystal habit. Cu++,V0- and U0 show appreciable influence on triglycine sulfate crystalhabit, but the results are yet to be correlated.

In growing the modified habit triglycine sulfate crystals, a small seedcrystal having the desired thickness is drilled and mounted on a glassrod so that the [101] faces of the crystal are perpendicular to the rod.The seed is then planted in a glass container containing approximately1400 ml. of saturated solution containing the required impurity additionat a temperature of 0.1 to 1.0 C. above the saturation temperature. Theglass container is immersed in a water bath controlled to i0.02 C. bymeans of a magnetic set, mercury thermoregulator and a sensitiveelectronic relay. The temperature is programmed down at rates varyingfrom 02 C. to 0.5 C. per day until the crystal reaches the desired size.Agitation within the solution is achieved by means of a magneticstirrer.

It is believed that this habit described above should apply as well tothe ferroelectric compounds of triglycine selenate, triglycinefluoberylate, and other similar compounds that are isomorphous totriglycine sulfate.

Changes in crystal habit are generally brought about by changes inone ormore of the following factors:

(1) pH of the solution (2) Rate of growth (3) Interaction between soluteand solvent (crystal habit may be changed depending on solvent used) (4)Concentration of growth active impurities a. Organic dyes b. Inorganiccations and anions Since the majority of laboratory controlled crystalhabit modifications reported in publications fall under item (4) above,this approach was attempted as well in subsidiary investigations oftriglycine sulfate. Several organic dyes were used as impurity additionsto saturated triglycine sulfate solutions in concentrations on a weightbasis ranging from 1 part dye: 8000 parts triglycine sulfate to 1 partdye: 1000 parts triglycine sulfate. The dyes used included Niagara SkyBlue BB,v Buffalo Black NBR, Trypan Red, Congo Red and Brilliant CresylBlue. Only minor habit modifications were obtained with any of the abovedyes, and none were of practical application. Mechanical methods ofinhibiting growth could be employed, such as placing the seed betweenglass plates, but this would conceivably lead to serious problems inobtaining uniform growth because of the difficulty of circulating thesolution uniformly about the seed.

This invention, the controlled habit modification of triglycine sulfateand deuterated triglycine sulfate by means of monovalent cations, hasresulted in low dispersion of the ferroelectric axis in single crystalplates thus grown and has permitted the fabrication of uniform thinsections from these plates, in turn permit-ting the building offerroelectric devices with greater uniformity of electrical propertiesand simplicity of fabrication and construction.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A method for habit modification and preparation of relatively largebut thin single crystal plates of triglycine sulfate and deuteratedtriglycine sulfate having highly desirable crystal habit for use inferroelectric devices, comprising:

(a) cutting a seed crystal of triglycine such that the distance betweenthe (101) faces of the crystal determines the thickness of the crystalto be grown,

(b) planting said seed crystal in a suitably agitated saturatedtriglycine sulfate solution containing at least one impurity forpreventing growth in the crystallographic [001] direction chosen from RbCS and Tl+ ions,

(c) programming the temperature of said saturated solution downward fora suitable period of time consistent with crystal growth until thecrystal grows to the desired size substantially only in two dimensionswith substantially no growth in the crystallographic [001] direction.

ZJThe method for habit modification and preparation of relatively largebut thin single crystal plates of tri- 30 glycine sulfate and deuteratedtriglycine sulfate having highly desirable crystal habit, comprising:

(a) mounting a seed crystalof triglycine on a support rod such that therod extends from one of the (101) faces of the crystal, the desiredthickness of the crystal to be grown being determined by the distancebetween the (101) faces of the seed crystal used,

(b) planting said seed crystal in a saturated triglycine sulfatesolution containing Tl+ ions as an impurity for effectively preventingcrystal growth inthe crystallographic [001] direction in a concentrationin excess of 1 gm.-atom '1'l+:l235 gm.-mols triglycine sulfate insolution,

(c) programming the temperature of said saturated solution downward fromthe saturation temperature at suitable varying rates per day and withminimum fluctuation of temperature consistent with the growth of asingle crystal until the crystal grows only in two dimensions to thedesired size,

ing the period of crystal growth, resulting in uniform growth of crystalplates substantially only in two dimensions with substantially no growthin the crystallographic [001] direction to produce thin single crystalplates whose ferroelectric axis lies in the plane of the crystal plateand its direction within the plate has a much smaller dispersion thancrystals of the same material having the normal habit.

3. The method for habit modification and prepara- 0 tion of relativelylarge but thin single crystal plates of triglycine sulfate anddeuterated triglycine sulfate having highly desirable crystal habit,comprising:

(a) mounting a seed crystal of triglycine on a support rod such that therod extends from one of the (101) faces of the crystal, the desiredthickness of the crystal to be grown beingdetermined by the distancebetween the (101) faces of the seed crystal used,

(b) planting said seed crystal in a saturated triglycine sulfatesolution containing Rb+ ions as an impurity for effectively preventingcrystal growth in the crystallographic [001] direction,

(c) programming the temperautre of said saturated solution downward fromthe saturation temperature at suitable varying rates per day and withminimum fluctuation of temperature consistent with the growth (d) andagitating said solution by suitable means dur-' of a single crystaluntil the crystal grows only in two dimensions to the desired size,

((1) and agitating said solution by suitable means during the period ofcrystal growth, resulting in uniform growth of crystal platessubstantially only in two dimensions with substantially no growth in thecrystallographic [001] direction to produce thin single crystal plateswhose ferroelectric axis lies in the plane of the crystal plate and itsdirection within the plate has a much smaller dispersion than crystalsof the same material having the normal habit.

4. The method for habit modification and preparation of relatively largebut thin single crystal plates of triglycine sulfate and deuteratedtriglycine sulfate having highly desirable crystal habit, comprising:

(a) mounting a seed crystal of triglycine on a support rod such that therod extends from one of the (101) faces of the crystal, the desiredthickness of the crystal to be grown being determined by the thicknessof the seed crystal used,

(b) planting said seed crystal in a saturated triglycine sulfatesolution containing Cs+ ions as an impurity for effectively preventingcrystal growth in the crystallographic [001] direction,

(c) programming the temperature of said saturated solution downward fromthe saturation temperature at suitable varying rates per day and withminimum fluctuation of temperature consistent with the growth of asingle crystal until the crystal grows only in two dimensions to thedesired size,

(d) and agitating said solution by suitable means during the period ofcrystal growth, resulting in uniform growth of crystal platessubstantially only in two dimensions with substantially no growth in thecrystallographic [001] direction to produce thin single crystal plateswhose ferro-electric axis lies in the plane of the crystal plate and itsdirection within the plate has a much smaller dispersion than crystalsof the same material having the normal habit.

References Cited UNITED STATES PATENTS 5/1961 Matthias 260534 X

1. A METHOD FOR HABIT MODIFICATION AND PREPARATION OF RELATIVELY LARGEBUT THIN SINGLE CRYSTAL PLATES OF TRIGLYCINE SULFATE AND DEUTERATEDTRIGLYCINE SULFATE HAVING HIGHLY DESIRABLE CRYSTAL HABIT FOR USE INFERROELECTRIC DEVICES, COMPRISING: (A) CUTTING A SEED CRYSTAL OFTRIGLYCINE SUCH THAT THE DISTANCE BETWEEN THE (101) FACES OF THE CRYSTALDETERMINES THE THICKNESS OF THE CRYSTAL TO BE GROWN, (B) PLANTING SAIDSEED CRYSTAL IN A SUITABLY AGITATED SATURATED TRIGLYCINE SULFATESOLUTION CONTAINING AT LEAST ONE IMPURITY FOR PREVENTING GROWTH IN THECRYSTALLOGRAPHIC (001) DIRECTION CHOSEN FROM RB+, CS+, AND T1+ IONS, (C)PROGRAMMING THE TEMPERATURE OF SAID SATURATED SOLUTION DOWNWARD FOR ASUITABLE PERIOD OF TIME CONSISTENT WITH CRYSTAL GROWTH UNTIL THE CRYSTALGROWS TO THE DESIRED SIZE SUBSTANTIALLY ONLY IN TWO DIMENSIONS WITHSUBSTANTIALLY NO GROWTH IN THE CRYSTALLOGRAPHIC (100) DIRECTION.